The present technique relates to medical imaging devices and, more particularly, to imaging devices with cryogenic cooling systems.
A number of important applications exist for superconductive magnet systems. These include imaging systems, as for medical imaging, as well as spectrometry systems, typically used in materials analysis and scientific research applications. The present technique relates to management of cryogenically cooled superconductive magnets, and particularly to the monitoring and servicing of such systems. Although reference is made throughout the following discussion to imaging systems, it should be borne in mind that the technique is applicable to a range of systems that utilize cryogenically cooled superconducting magnets.
Imaging devices are omnipresent in typical medical and other environments. Medical practitioners, such as physicians, may employ medical imaging devices to diagnose patients. Imaging devices, such as Magnet Resonance Imaging (MRI) devices and Nuclear Magnetic Resonance (NMR) devices, produce detailed images of a patient's internal tissues and organs, thereby mitigating the need for invasive exploratory procedures and providing valuable tools for identifying and diagnosing disease and for verifying wellness.
Typical MRI and NMR devices develop diagnostic images by affecting gyromagnetic materials within a patient via controlled gradient magnetic fields and radiofrequency pulses in the presence of a main magnetic field developed by a superconductive magnet. During an MRI exam, a main magnetic field of upwards of two Tesla may be necessary to produce vivid images. Typically, superconductive electromagnets comprise loops of coiled wire, which are continuously bathed in a cryogen, such as liquid helium, at temperatures near absolute zero. For the example of bathing the coils with a liquid pool of helium, system temperatures are approximately −269° C. (or 4 K) near atmospheric pressure (e.g. less than 5 psig). When cooled to such extreme temperatures, the coiled wire becomes superconductive, i.e., the electrical resistance of the wire falls to essentially zero, enhancing the field strength without requiring significant energy input for continued operation. Advantageously, superconductive electromagnets reduce the electrical load requirements for producing the desired magnetic fields, thereby making the MRI system more economical to operate.
Challenges exist, however, in maintaining the electromagnets at these extreme temperatures which are significantly lower than ambient temperatures. Because of this temperature difference with ambient, a considerable driving force exists for heat transfer from the environment into the magnet system. Accordingly, thermal insulating material and other heat transfer barriers, such as vacuum regions, may insulate the magnet and cryogen to impede heat transfer from the environment. For environmental heat effects that reach the inner workings of the magnet system, the liquid pool of cryogen that surrounds the magnet may absorb the heat to maintain the magnet at desired temperature. Cryogens operating at or near their boiling points typically expend this external heat by vaporizing relatively small amounts of cryogen.
In general, the cryogen liquid pool and its heat of vaporization consume heat while maintaining the magnet at constant temperature. On the whole, cryogen liquid pools in well-insulated systems, such as typical superconducting magnet systems, are able to absorb heat transferred from the environment over relatively long periods of time to maintain the magnet at desired temperature. Cryogenic liquids, such as liquid helium, however, are relatively expensive to produce and maintain. Therefore, older approaches of “open” systems which have no recondensing capability and where cryogen vapor is normally vented to the atmosphere, have generally fallen out of favor in the industry. In these systems, as the liquid cryogen absorbs environmental heat in maintaining the desired magnet temperature, vaporized cryogen is normally vented to limit pressure increases and thus to limit temperature increases.
Accordingly, to conserve cryogen, such as helium, and to support cryogen pressure control, magnet systems in typical MRI devices may now include a cryogen condensing system, which recondenses volatilized cryogen back into its liquid phase. That is, cryogen is maintained in a sealed cryogen vessel (or cryostat) that provides cryogen vapor (i.e., gaseous helium) to the condensing system and receives liquid cryogen (i.e., liquid helium) from the condensing system in a closed loop process. The condensing system condenses cryogen vapor, thus recovering the vapor, as well as, maintaining the cryogen pressure below the set point of the vent or relief device. In contrast, for low operating pressures, as discussed more below, a heater may be used to prevent the cryogen pressure from dropping too low. In sum, for the older open systems, a loss in cryogen level is expected and the timing of service intervals is typically based on this loss of level. In contrast, for recondensing magnets systems which recover the vaporized cryogen, losses in cryogen liquid level are not expected during normal operation. Thus, recondensing magnet systems generally retain cryogen level and reduce the requirement of periodic refilling of cryogen.
Recondensing magnet systems, however, from time to time, require maintenance, for example, when the cryogen condensing system may require repair or replacement. In particular, the performance of the condensing system components may degrade, thereby reducing the efficacy of the condensing system and overall magnet cooling system (cryogenic cooling system). Moreover, leaks within the cryogen (helium) vessel and/or condensing system, again for example, may also reduce the efficacy of the cooling system. During maintenance, it may become necessary to disengage the condensing system, cooling system, and/or deactivate the MRI devices, events that are to be avoided. If the cryogen condensing system is off-line or not condensing effectively, more of the liquid cryogen may begin to volatilize, leading to an increase of pressure in the cryogen vessel (i.e., cryostat). To prevent adverse effects due to the increased pressure, traditional MRI devices, such as a relief valve installed, for example, on the cryogen vessel may relieve pressure by venting some of the gaseous cryogen to the atmosphere. This conversion of liquid cryogen, such as liquid helium, to its gaseous state, and/or the subsequent venting of the gas, is generally known in the industry as “boil-off.” Venting of the gas leads to permanent loss of expensive cryogen, requiring periodic refilling of the system. Again, this venting is expected during normal operation of open systems but may be substantially avoided in recondensing systems.
Recondensing technology may provide “zero boil-off” systems in which a refrigeration system or “cold head” (also called cryo-cooler or cryo-condenser) typically runs continuously to condense (and recondense) vaporized cryogen. An electric heater in the vessel heats the cryogen to maintain a desired pressure level, thereby preventing the vessel pressure from falling below a desired level that could result in drawing atmospheric gases into the vessel. A balance is maintained between cooling components (condensing) and heating components (vaporizing) which can be continuously monitored. In contrast, with “open loop” thermal designs, such as those without recondensing technology, the expected boil-off results in a measurable drop in liquid level in the vessel over time. In some configurations, this drop in liquid level may manifest in a reduction in the reading, for example, on a provided liquid level gauge. Historically, with open systems, the percent rate of drop in a volumetric table unique to each magnet system (i.e., in each MRI system) defines the boil-off rate, for example, expressed as liters per hour consumption. For open systems, this boil-off rate is compared to design norms to determine total thermal system performance and used to determine service intervention. On the contrary, recondensing systems experience minimal loss of cryogen level, employing the refrigerating action of a cold head to provide controlled zero boil-off (COBO) with high percentage cryogen (helium) recovery. Though true zero boil-off systems are not physically possible, the operation of condensing the helium vapor and returning the condensed helium to the helium vessel (cryostat) liquid pool may approach ideal conditions.
Conventional methods of evaluating thermal system performance depend upon observing a drop in liquid level and then comparing the results to design norms. Such approaches are proving to be generally inadequate for recondensing systems. During normal and early failure modes, recondensing thermal systems, by design, exhibit no drop in liquid level over time thereby preventing prompt identification failure until the problem becomes severe enough, for example, that the pressure relief valve opens the previously closed system and gas is vented resulting in the liquid level dropping. It should be emphasized that improved monitoring and indication of magnet performance and operating variables are needed to recognize and respond to problems affecting recondensing magnet thermal performance, magnet superconductivity and field strength, MRI image quality, and so forth. Such monitoring of recondensing magnet systems is needed to facilitate predictive maintenance and effective scheduling of MRI service intervals, and to proactively reduce helium boil-off, loss of superconductivity, equipment damage, maintenance costs, MRI down time, and the like.
Traditionally, the maintenance of cryogenic cooling systems in MRI devices is a reactive process. That is, technicians are generally called when, for example, image quality has been affected, a critical indicator has activated, and/or the system is no longer operable. For example, a typical system may generate a service call when a low level of cryogen is detected due to venting or leaks in the system. In addressing concerns reactively, the repair time and/or off-line periods may be longer than desired. For example, certain parts and/or technicians may not be immediately available, leading to longer than necessary downtimes (i.e., off-line time). Moreover, periods of reactive maintenance may not coincide with already scheduled routine maintenance procedures, leading to duplicative downtimes for the MRI device. Similarly, when substantial quantities of cryogen are required, very significant costs may be incurred in refilling the serviced system.
Similar problems exist even prior to the time such magnets are placed in operation. For example, magnets are typically built and tested in a controlled factory environment, then at least partially disassembled from other support equipment for shipping. Current procedures for building, testing and shipping superconductive magnets do not, however, adequately accommodate boil-off or servicing needs. In much the same way, mobile MRI systems and systems where communications infrastructures are less available pose particular challenges beyond those of traditional fixed locations in hospitals. Such challenges include cryogen monitoring and servicing; but also location and identification of the systems, and communication of relevant parameter data to a monitoring or service-coordinating location.
Accordingly, there is a need for an improved technique for maintaining cryogenic cooling systems. Particularly, there is a need for a technique that reduces maintenance times, periods of deactivation, costs, and so forth.